1. Field of the Invention
The present invention relates to radio modulators. More particularly, the present invention relates to a radio modulator that scan the FM broadcast band, searching for a transmit frequency with low ambient signal interference levels on which to subsequently transmit a modulated FM signal.
2. Description of the Related Art
Radio modulators are used to modulate base-band signals onto a radio carrier, and then transmit the modulated carrier to a nearby radio receiver. There are several types of commercial FM modulators available in the market, which typically operate as “Part 15” lower power devices. Such products are commonly called FM modulators or FM transmitters. FM modulators are employed to provide a wireless interface between the analog audio output of a portable audio playback device and an FM broadcast radio receiver. A common application for this type of product is to couple the audio output from an MP3 audio player to the FM radio receiver in an automobile, although many other applications are known.
The transmit frequency used by an FM modulator may be fixed or selected from a small number of predetermined frequencies, such as four or eight channels. Other prior art FM modulators allow selection of any of the one hundred FM station frequencies (i.e. 88.1 MHz to 107.9 MHz) allocated in the FM broadcast band. In each of these prior art FM modulators, the user manually selects the transmit frequency. A significant problem with this approach is that the frequency selected could be the same as, or close to, the frequency of the local FM broadcast radio station. As a result, detrimental interference occurs, which degrades the quality of the audio coupled from the FM modulator to the local FM radio receiver. The technique for avoiding such interference is to select a transmit frequency within the FM band that has a low ambient signal level, so the chance of interference is minimized. Unfortunately, this technique is frequently not understood or properly utilized by the casual end user of such products. Even in the case where the end user understands this selection process, the interference environment changes gradually as the FM modulator and radio move to a different geographic location.
The prior art has addressed the foregoing issue by implementing FM modulators that have an additional radio receiver circuit, which is used to check for ambient radio interference, and then recommends a transmit frequency to the user. For example, U.S. Pat. No. 5,970,390 to Koga et al. for TRANSMITTER AND AUTOMOBILE AUTO APPARATUS USING THE SAME teaches an FM transmitter that includes a transmitter circuit for converting the audio signal derived from an audio appliance into an FM signal within an FM broadcasting band to thereby transmit the FM signal. Kogal et al. includes a receiver circuit with a tuner unit, a display for displaying thereon a frequency, and a control circuit for controlling a transmission frequency of the transmitter circuit and the reception frequency of the receiver circuit. The control circuit causes the receiver circuit to detect an unused frequency within the FM broadcasting band and to set a frequency corresponding to the detected unused frequency to the transmission frequency, and also causes the display to display thereon the set transmission frequency. The implementation of this teaching requires a full receiver and a full transmitter circuit. Also, Koga et al. do not teach scanning the entire FM radio band, rather it teaches that the receiver increment until a suitably quiet frequency is identified. An absolute signal level threshold is therefore used as a reference.
Another prior art teaching is U.S. Pat. No. 6,782,239 to Johnson et al. for WIRELESS OUTPUT INPUT DEVICE PLAYER. Johnson et al. teach a wireless output and input device digital audio player that stores and/or receives digital audio, and then translates the stream to analog prior to modulation and transmission. The Johnson et al. device includes a full FM receiver circuit and a full FM transmitter circuit for transmitting the audio to an external FM radio or receiver. The device automatically and periodically scans a plurality of channels on the FM band to determine a channel having the least amount of interference and then tunes the FM transmitter to the frequency of that channel and displays the channel to which the external receiver should be tuned. The Johnson et al. device scans about fifteen frequencies. Signal quality is determined using a signal to noise ratio (“SNR”) test that integrates the received noise over time and compares that with the peak signal level during that time period to establish an SNR value, which is stored. This takes considerable time, and requires an absolute threshold level detection process.
While the prior art teachings address certain issues regarding the selection of a suitable transmit frequency for use in an FM modulator, there are other issues remaining. The prior art teachings require the implementation of a full receiver circuit in addition to the transmitter circuit. This increases circuit complexity, cost and size of the apparatus. The prior art also employ absolute signal threshold level detection technique that doesn't adapt well to the vagaries of the radio environment, such as a city versus a rural environment. Thus it can be appreciated that there is a need in the art for an improved radio modulator that scans the entire radio band seeking the best channel or frequency on which to transmit.